1. Field of Invention
This invention relates to a method and apparatus for separating a mixture of objects according to the density of the individual objects. This invention further relates to means for separating solids of varying densities by use of a fluid separation column. More particularly, the present invention is concerned with a novel separating technique and apparatus in which solid articles having slightly differing densities may be separated by use of a fluid separation column in which density is altered in order to cause the solids of a higher density to be separated from the solids of a lower density due to positive buoyancy of the lower density solids and negative buoyancy of the higher density solids. This invention also relates to a novel method for altering the bulk density of a fluid separation medium. 2. Prior Art
For separation of larger objects such as produce, including potatoes and other vegetables, the prior art teaches a buoyancy method of separation in which the rate of ascent of the submerged objects in a brine or other liquid solution is utilized to perform the separation. The produce is injected into a moving current of liquid solution which has a density intermediate to the densities of the objects to be separated. The lower density objects surface first and are removed separately from the higher density objects. This prior art process is limited by the fact that the density of the liquid separating medium cannot be altered or adjusted readily.
Another method for separating larger objects is shown in U.S. Pat. No. 3,822,015 (Hsieh, et al.). The Hsieh patent teaches a method of separating materials of different densities by altering the apparent density (i.e., bulk density) of a fluid separation column. In this prior art method the materials to be separated are placed in the fluid separation column wherein they seek a height position dictated by the buoyancy within the fluid separating column (i.e. a position of neutral bouyancy). The apparent (i.e., bulk) density of the fluid separation column is then changed to a value intermediate to the lower density fraction and the higher density fraction of the materials being separated. The higher density fraction then sinks to the bottom of the fluid separation column due to the negative buoyancy, and the lower density fraction remain afloat. The Hsieh patent teaches certain methods for altering the bulk density of the fluid separation column, including, sparging a gas into the fluid and adding soluble chemicals, immiscible fluids, or other fluids.
The method and apparatus taught by the Hsieh patent have been plagued with several difficulties which have, up to now, prevented commercial success. These difficulties include, hydraulic instability, relative inflexibility and insensitivity of the system, air bubble control problems, geometrical constraints, undesirable frothing problems when chemical additives are present, and maintaining a fixed or constant separation column bulk density.
The present invention successfully overcomes each of the above-stated problems and provides numerous additional advantages which will be apparent from the discussion below.
This invention has as one object the provision of an improved process and apparatus for separating relatively large mesh sized articles according to their density by use of a fluid separating column. A further object of this invention is to provide a process and apparatus in which macroscopic sized solids are separated according to their densities by use of a fluid separating column in which the effective density of the fluid separation medium is varied, causing the higher density objects to have a negative buoyancy and the lower density objects to have a positive buoyancy. Another object is to provide a convenient means for varying the effective density of the liquid separation column such that the cut point for separation can be easily controlled.
Another object of this invention is to provide an improved method and apparatus for causing separation of solids according to their densities using a variable density fluid separating column. A related object is to provide apparatus which causes a variation in the apparent density of the fluid separating column so as to cause the lighter density fraction of the solid articles to float on the fluid separating medium and the heavier density articles to sink.
A still further object of this invention is to provide a more convenient and more sensitive means to alter the effective density of a fluid. A further related object is to provide means to alter the effective density of fluids used in solids handling apparatus to effect a separation of the solids being treated in the apparatus simultaneously with the solids handling and transporting operation.